Although great importance has been attached to the study of the toxicity of silica, mainly the involvement of silica in silicosis, there had been little work concerned with the effect of silicon in normal metabolism and there was no evidence that there was any need for silicon in higher animals until silicon was reported to be an essential trace element in 1972 as a result of earlier studies in this laboratory. Silicon was shown to be required for growth and skeletal development in the chick. The increase in growth was marked suggesting that silicon must play an important role in the body's metabolic processes. Our later work shows that silicon must play an important role in normal functioning of connective tissue as we have shown silicon to be involved in several aspects of connective tissue metabolism such as (1) in connective tissue formation (2) as a structural component (3) in aging of certain connective tissues (4) in bone calcification processes and recently (5) at the cellular and subcellular level. We propose therefore to make an intensive investigation into the role of silicon in connective tissue metabolism with additional emphasis on aorta and bone. Silicon's role seems particularly striking in (1) the walls of the aorta, where we have found a dramatic decline of silicon content with age and in (2) bone, where our earliest studies implicating silicon as an essential element were relating to calcification; moreover, recent findings at both the cellular and subcellular level in osteogenic cells call for an investigation of silicon's role in both cell and mitochondria basic to an understanding of bone metabolism including calcification, calcium homeostasis and an understanding of the mitochondria themselves. In vivo studies using chicks and rats and employing the conditions developed in this laboratory to produce a silicon deficiency in the young animal, and in vitro studies including isolation and quantitative analyses of connective tissue components, electron microprobe and tissue culture will be carried out to attain these objectives.